1. Technical Field
Integrated semiconductor waveguide gratings are disclosed that are fabricated using ion implantation. Methods of fabricating integrated semiconductor waveguide gratings are disclosed that use ion implantation to form a refractive index grating along the length of the integrated semiconductor waveguide. Further, methods of apodizing integrated waveguide gratings are also disclosed.
2. Description of the Related Art
There is a wide-ranging demand for increased communications capabilities, including more channels and greater bandwidth per channel. The needs range from long distance applications such as telecommunications between two cities to extremely short range applications such as the data-communications between two functional blocks (fubs) in a semiconductor circuit with spacing of a hundred microns. Optical media, such as optical fibers or waveguides, provide an economical and higher bandwidth alternative to electrical conductors for communications. A typical optical fiber includes a silica core, a silica cladding, and a protective coating. The index of refraction of the core is higher than the index of refraction of the cladding to promote internal reflection of light propagating down the silica core.
Optical fibers can carry information encoded as optical pulses over long distances. The advantages of optical media include vastly increased data rates, lower transmission losses, lower basic cost of materials, smaller cable sizes, and almost complete immunity from stray electrical fields. Other applications for optical fibers include guiding light to awkward places (e.g., surgical applications), image guiding for remote viewing, and various sensing applications.
The use of optical waveguides in circuitry to replace conductors separates path length affects (e.g., delays) from electrical issues such as mutual impedance. As a result, optical interconnects and optical clocks are two applications for waveguide technology.
The signal carrying ability of optical fibers is due in part to the ability to produce long longitudinally-uniform optical fibers. However, longitudinal variations in index of refraction, e.g., those associated with refractive-index gratings, can be included in the optical fibers to affect the transmitted pulses in useful ways. Gratings can be grouped into short-period, e.g., about 0.5 micron (xcexcm), or long-period, e.g., about 200 xcexcm, gratings. Short-period gratings can reflect incident light of a particular wavelength back on itself in the fiber. Short-period gratings are also called Fiber Bragg Gratings. Long-period gratings can couple incident light of a particular wavelength into other co-propagating modes of the fiber or selectively block certain wavelengths from propagating efficiently through the fiber.
Apodized Fiber Bragg Gratings are commonly used in wavelength division multiplexing optical networks while waveguides and photonic band gap crystal structures are used in integrated optical-electronic devices without apodization. Apodization can be utilized in Fiber Bragg Gratings to either attenuate light of certain wavelengths or narrow the bandwidth passing through the grating by either absorption or reflection of light at undesired wavelengths. Fiber Bragg Gratings are apodized using hydrogen loading systems or ultraviolet light sources to modify the refractive index along the length of the grating. One advantage of Fiber Bragg Gratings is that they are made within the fiber itself which minimizes insertion loss, simplifies manufacture and eliminates the need for complex and precise alignment. However, because Fiber Bragg Gratings are created within the fiber itself, the applications are limited and therefore cannot be used in coupling systems, routing systems or opto-electronic devices. The use of ultraviolet UV light for bleaching purposes in a refractive index grating requires a guide medium that does not unacceptably absorb the UV light.
Photonic band gap crystal structures are engineered with uniform or periodic changes in their dielectric constant, analogous to the crystal structure of semiconductors, which creates a band gap for photons or a range of frequencies where electromatic waves cannot exist within the material. As a result, photonic band gap crystal structures allow control of the frequencies and directions of the propagating light. Unfortunately, apodization is not currently possible with uniform photonic band gap crystal structures. More importantly, because integrated waveguides and photonic crystals are often fabricated in semiconductors that absorb ultraviolet light, bleaching is not a practical means for generating index of refraction changes.
Thus, there is a need for gratings that can be employed in optical-electronic devices, like the photonic band gap crystal structures, but which can be apodized, if desirable, like a Fiber Bragg Grating.